1. Field of the Invention
The invention relates to a newly developed class of spiro orthocarbonate (SOC) monomers which can undergo double ring-opening polymerization with an expansion in volume. These new monomers offer significant improvements over previously known SOC monomers and have particular utility in compositions including conventional dimethacrylate dental monomers. The SOC monomers of the invention undergo double ring-opening polymerization where two bonds are broken for each new bond formed with a resulting expansion in volume that counters the volume contractions associated with shrinkage of the resinous component(s) of the composition. The resulting polymerized resins exhibit higher adhesive strength to the substrates to which they are bonded.
2. Description of the Prior Art
A wide variety of material deficiencies have been attributed to the volume contraction associated with the polymerization of resins in composite and adhesive applications, especially in the field of dental resins. These problems arose from the generation of stresses which are of sufficient magnitude to cause defects within the resin matrix and debonding at the interfaces. See Feilzer et al., "Setting Stress in Composite Resin in Relation to Configuration of the Restoration", J. Dent Res, 66:1636-1639 (1987); Feilzer et al., "Quantitative Determination of Stress Reduction by Flow in Composite Restorations", Dent Mater, 6:167-175 (1990); and Zindan et al., "A Comparative Study of the Effects of Dentinal Bonding Agents and Application Techniques on Marginal Gaps in Class V Cavities", J. Dent Res, 66:716-721 (1987). These complications undermine performance and often restrict those situations for which compositions and other related dental resin materials can be considered. One approach toward minimizing the extent of polymerization shrinkage in resins is to incorporate free-radically polymerizable spiro orthocarbonate monomers into formulations with conventional dimethacrylate dental monomers. The spirocyclic monomers can undergo double ring-opening polymerization wherein two bonds are broken for each new bond formed. The result is an expansion in volume that can be applied to counter the volume contraction.
While double ring-opening polymerization of SOC monomers has previously been disclosed in U.S. Pat. No. 4,387,215 to Bailey, the utility of those types of monomers in free radical polymerization has been severely limited by their relative lack of reactivity with conventional comonomers and their reluctance to undergo the desired ring-opening process when they do react. Other experience with spiro orthocarbonate monomers formulated into dental resins has demonstrated some distinct advantages. Thompson et al., "Dental Resins with Reduced Shrinkage During Hardening", J. Dent Res, 58:1522-1532 (1979) evaluated a relatively high melting, symmetrical spirocyclic monomer included as a slurry in resin compositions. The melting and subsequent ring-opening of the spiro monomer during the polymerization yielded a nearly volume-neutral curing process overall and resulted in a doubling of the adhesive strength of the resin to etched enamel as compared with a control. However, a large proportion of the crystalline spiro component was left undissolved and unreacted in the resin cured at ambient temperature.
These early results led to the development of free-radically polymerizable, asymmetric spiro orthocarbonate monomers, See Endo et al., "Synthesis and Radical Ring-Opening Polymerization of Spiro o-Carbonates", J. Polym. Sci, Polym Chem Ed, 13:2525-2530 (1975) and Stansbury et al., "Synthesis of Monomers that Polymerize with Expansion in Volume", J. Dent Res, 65:219, Abst. No. 452 (1986), which can have melting points below room temperature depending on substituent and ring size considerations. Liquid spirocyclic monomers were substituted in place of the dimethacrylate diluent monomer in dental composite formulations, See Stansbury et al., "Evaluation of Spiro Orthocarbonate Monomers Capable of Polymerization with Expansion as Ingredients in Dental Composite Materials", Progress in Biomedical Polymers, C. G. Cabelein and R. L. Dunn, Eds., New York: Plenum Press, pp. 133-139 (1990). These experimental materials provided polymerization shrinkage values as much as 40% less than that of the control along with a corresponding three-fold increase in the adhesive strength measured between the composite and stainless steel. The disclosures of each of the foregoing publications are herein incorporated by reference in their entirety.
The use of spiro orthocarbonate compounds in combination with resins has not been limited to dental resins. Yamamoto et al. in U.S. Pat. No. 4,980,109 (the entire disclosure of which is herein incorporated by reference) describe insert injection molding processes utilizing SOC compounds with various molding resins including various kinds of rubber, such as butadiene, isoprene, nitrile, chloroprene and acrylic rubbers; various elastomers such as polyurethanes, polyester, polyamide, and polyolefin elastomers; various thermoplastic resins such as thermoplastic polyurethane, polyamide, polyester, polycarbonate, polystyrene, polyacrylate, polyvinyl alcohol, polyvinyl acetate, and polyvinyl chloride; and thermosetting resins such as epoxy, phenolic, melamine, and unsaturated polyester resins.
It was in light of the long felt need in the art and the numerous attempts by other researchers to provide a suitable class of monomers which undergo expansion in polymerizable compositions by which the current investigation was undertaken in an effort to develop a new generation of improved monomers which ideally would meet the following criteria:
The monomers should have sufficient reactivity toward free radical addition so that active participation in copolymerizations with monomers, such as methacrylate, is guaranteed;
The monomers should efficiently utilize the available double ring-opening pathway at the temperature in which they are used, especially at ambient or near ambient temperatures, dictated especially by dentistry; and,
The monomers should contribute a sufficient glass transition component to the copolymer, through the use of substituents or cross-linking, so that large proportions of such monomers can be utilized to control shrinkage without significantly compromising the overall properties and performance of the resulting materials.